How to composite in a background picture?
Another option is an Alpha over node:




Add an Image node (ShiftA> Input > Image) and open your background image:


To crop or scale your background image to fit your render, add a Scale node (ShiftA> Distort > Scale).
Set the Coordinate Space to Render Size, and set the scale type. If your background image is larger than the render, you probably want to crop it. (select Crop) However, if it is smaller and you want to scale it up, select Fit:


Connect the nodes as shown above.